This invention relates to optical system components, and in particular, to retarders.
A typical retarder is a plate made of a material in which the speed of light depends on the polarization of that light. Such materials are known as xe2x80x9cbirefringentxe2x80x9d materials. A birefringent material resolves an incident light wave into a slow wave, corresponding to one component of the incident light wave""s polarization vector, and a fast wave, corresponding to another, orthogonal component of that wave""s polarization vector. The slow wave travels at a slower velocity than, and is therefore retarded relative to, the fast wave. As a result, the wave that emerges from the birefringent material can have a polarization state that differs from that of the wave incident on the material.
The xe2x80x9cretardancexe2x80x9d of a retarder is a distance by which the slow wave is delayed relative to the fast wave. Commonly used retarders are those having retardances of (n+xcex/4) and (n+xcex/2), where xe2x80x9cnxe2x80x9d is an integer that describes the xe2x80x9corderxe2x80x9d of the retarder. For precision applications, in which stability with wavelength, temperature, and angle of incidence are critical, zeroth order retarders are desirable.
A commonly used birefringent material for making a retarder is quartz. However, the birefringence of quartz is such that only a very thin layer is required to achieve a zeroth order retarder. Such thin layers of quartz are notoriously fragile. In addition, quartz based retarders are expensive and difficult to manufacture in larger sizes.
Birefringent polymers overcome many of the disadvantages of quartz. However, such materials come as flexible films that must be mounted and glued between rigid plates. During the mounting process, the polymer films are prone to wrinkle. This wrinkling causes wavefront errors that are undesirable in high-precision applications.
In one aspect, the invention features a retarder that includes a substrate and a cover plate separated from the substrate by a gap. The gap is occupied by an adhesive filler that has, embedded within it, a birefringent film. The birefringent film and the adhesive filler are index matched to each other, thereby reducing reflections at the interfaces between the film and the adhesive.
In another aspect, the invention features an interferometer having a polarizing beam splitter cube and a cover plate separated from a face of the cube by a gap. Embedded within an adhesive filler that occupies the gap is a birefringent film. The birefringent film and the adhesive filler are index matched to each other.
Another aspect of the invention features a retarder that includes a birefringent film embedded in an adhesive. The adhesive and the film are index matched each other.
Embodiments may include one or more of the following. An anti-reflective coating may be on a surface of at least one of the cover plate and the substrate. Such a coating may include thorium fluoride. At least one of the cover plate and the substrate may include fused silica. The birefringent film may include a polymer film.
The substrate and the cover plate can each have an inner surface facing the gap. These inner surfaces may be parallel to each other or oriented to direct a beam exiting the cover plate in a selected direction. Alternatively, the cover plate and the substrate may be disposed relative to each other to direct a beam exiting the cover plate in a selected direction.
Additional embodiments include those in which an optical device is in optical communication with a retarder or integrated with a retarder as set forth above. Examples of such optical devices include interferometers.
Embodiments of the invention may have one or more of the following advantages. The retarder provides low wavefront distortion, including both distortion introduced by optical elements and distortion introduced by beam shear. This results, in part, from the retarder""s high thermal and mechanical stability, high tolerance retardation characteristics, and deterministic angular deviation of a beam exiting the retarder. A high stability plane mirror interferometer having an integral retarder according to the invention has fewer parts and fewer ghost reflections. This translates into a reduction in associated cyclic errors that can degrade performance.
These and other features and advantages of the invention will be apparent from the following detailed description and the accompanying drawings, in which: